1. Field of the Invention
The present invention relates to a method for manufacturing a metallic material having a composite metal structure consisting of a non-ferrous metal alloy such as magnesium alloy or aluminum alloy and a carbon nanomaterial.
2. Description of the Related Art
The carbon nanomaterial that is one kind of crystalline carbon materials has characteristics such as having about 5 times higher heat conductivity than non-ferrous metals such as aluminum (Al) and magnesium (Mg), satisfactory electric conductivity, and excellent slidability due to low friction factor. However, the carbon nanomaterial is recommendable to be made into a composite by mixing with other substances in application thereof because it is an ultrafine material of nm scale.
A conventionally known art for compositing a metallic material with a carbon nanomaterial comprises kneading the carbon nanomaterial with a metal powder followed by pressure refining to form composite material grains having a metal powder grain size of 5 μm to 1 nm, and the composite material grains are thermally compressed by hot press molding, and processed into a product consisting of a composite metallic material. However, since the product shape is restricted in this product processing by hot press molding, this method stops short of manufacturing a metal product such as a heat radiating part or shield part for electronic equipment or a bearing which was difficult to manufacture by press molding.
Therefore, it has been tried to form a composite metallic material adaptable for a metal molding machine by perfectly melting a metallic material to a temperature of a liquidus temperature or higher, adding carbon nanomaterial to the metallic material of in the liquid phase state, and stirring and kneading the metallic material with the carbon nanomaterial by a stirring machine. However, since the carbon nanomaterial is poor in wettability with the metallic material in the liquid phase state and is difficult to be dispersed uniformly in the liquid phase due to floating by stirring, this method has not been put into practical use, so far.
As a new means for uniformly dispersing the carbon nanomaterial, it has been performed to cool a molten metallic material from a liquid state into a semi-solid state, spheroidize granular solid phase in a liquid phase which is generated in this cooling process to form a semi-solid metallic material showing thixotropic properties, and add the carbon nanomaterial thereto followed by stirring and kneading. Although this spheroidization of solid phase is performed by flowing down the metallic material over the plate surface of an inclined cooling plate in a molten state thereof, the spheroidization can be performed also by adding a crystal grain refining agent or by applying an electromagnetic vibration force or an ultrasonic vibration force.
Patent Literature 1: Japanese Patent Application Laid-Open No. 2004-136363
Patent Literature 2: Japanese Patent Application Laid-Open No. H06-73485
Patent Literature 3: Japanese Patent Application Laid-Open No. 2004-98111
Although the dispersion of the carbon nanomaterial is enhanced in the above-mentioned compositing with the metallic material in the semi-solid state, compared to the compositing with the metallic material molten to the liquid phase state, part of the carbon nanomaterial is left in lumps in the liquid phase between solid phases as it is coagulated. This is caused by the fact that the carbon nanomaterial itself is easy to coagulate, and the dispersion is limited to the liquid phase between solid phases, and the coagulation cannot be entirely broken and dispersed by the stirring by rotation of a stirring blade, and homogenization of the composite metal structure had its limit. When ultrasonic vibration is adapted as a stirring means by vibration, the carbon nanomaterial floats on the surface layer of the semi-solid metallic material by the vibration, and mostly left in the upper layer, and the resulting difference in density of the carbon nanomaterial between the upper layer and the lower layer makes it difficult to bring the composite metal structure into a homogenous state.